Infection of Normal Rat Kidney (NRK) cells by Rous Sarcoma Virus (RSV) will be investigated in vitro. The viral genes responsible for efficient infection of mammalian cells will be identified and those affecting viral envelope properties will be defined by phenotypic mixing experiments with vesicular stomatitis virus. Somatic cell hybridization between NRK cells infected with different temperature sensitive mutants of RSV will be employed to study regulation of virus expression and complementation between the viral mutants. Viral mutants for both transformation and replication will be classified into complementation groups. The possibility of complementation between virus strains will be studied. Virus rescued from the hybrid clones will be analyzed to study the mechanism of viral recombination and to map possible viral linkage groups. BIBLIOGRAPHIC REFERENCES: Boettiger, D., Love, D.N., Weiss, R.A.: Virus Envelope Markers in Mammalian Tropism of Avian RNA Tumor Viruses. J. Virol. 15:108-114, 1975. Boettiger, D.: Activation and Repression of Virus Expression in Mammalian Cells Infected by Rous Sarcoma Virus. Cold Spring Harbor Symp. Quant. Biol. 39:1169-1172, 1974.